Guest commentary by David Middleton

From the No Schist Sherlock files…

PUBLIC RELEASE: 19-APR-2018

Humans have been driving a global reduction in mammal size for thousands of years

AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE

The dispersal of humans out of Africa coincided with a dramatic global reduction in the size of mammals, a new study reveals. This “downsizing” trend may continue, suggest the authors, to the extent that, in just a couple hundred years, the largest terrestrial mammal left may be the domestic cow, weighing in at 900 kilograms (kg). Felisa A. Smith et al. sought to understand how the size of mammals has changed over time. They updated and created two datasets that capture the global distribution and body size of terrestrial mammals that lived between 66 million years ago through the present. The authors found a substantial bias in mammal extinction during the periods when humans were dispersing around the globe, whereby species that went extinct tended to be two to three times bigger than mammals that survived, a trend that was evident globally. Notably, prior to humans’ migration out of Africa 125,000 years ago, Africa was home to mammals of smaller size (with a mean body mass roughly half that of mammals found in Eurasia), which the authors suggest is reflective of the hominin-mammal interactions that had already been at play. Perhaps most striking is the reduction of mammals in the New World during the late Pleistocene, which coincided with humans’ adoption of long-range weapons. The authors report a greater than 10-fold drop in both mean and maximum body mass of mammals during this time; for example, mean mass of terrestrial mammals in North America fell from 98.0 to 7.6 kg. If current trends continue, the mean body mass of mammals in North America will drop from 7.7 to 4.9 kg in a few hundred years, the authors say. As mammals play a critical role in shaping ecosystems, the downsizing trend will have a cascading impact on other organisms.

Eureka Alert

The paper is pay-walled.  Here are a few highlights from the abstract:

1. Today, it is well known that human activities put larger animals at greater risk of extinction. 
2. If the current trend continues, terrestrial mammal body sizes will become smaller than they have been over the past 45 million years.
3. Since the late Pleistocene, large-bodied mammals have been extirpated from much of Earth. 
4. This decline is coincident with the global expansion of hominins over the late Quaternary. 
5.  Moreover, the degree of selectivity was unprecedented in 65 million years of mammalian evolution.
6. The distinctive selectivity signature implicates hominin activity as a primary driver of taxonomic losses and ecosystem homogenization.

And my comments on the highlights:

1. They were in the way, some of them hunted us and most of them tasted good.
2. Where do you think Chihuahuas, Pomeranians and Corgis came from?  Besides, when was the last time a large mammal species became extinct?  What percentage of megafauna bought the farm in the Early Holocene vs the fictitious “Anthropocene”?
3. I’m sure we didn’t intend to extirpate most of them.  My guess is that most of them were long gone before the word “extirpate” was invented.  It’s highly likely that our ancestors used the word “eaten” in the early days of language.
4. Are you saying we should have stayed in Olduvai Gorge?  Or that we should have extirpated ourselves?
5. Blame the dinosaurs.  If they didn’t get extirpated. none of this would be happening.
6. So what?

Is “the bleeding obvious” really a scientific discipline?  Humans played a big role in wiping out 169 out of 244 genera of mammalian megafauna from the Late Pleistocene through the Early Holocene.  This isn’t a eureka-worthy new discovery:

Mammoths, Stegodons and Mastodons loved the Plio-Pleistocene but never got acquainted with the Holocene. I wonder why?

Here’s another gem from the Eureka Alert article:

Perhaps most striking is the reduction of mammals in the New World during the late Pleistocene, which coincided with humans’ adoption of long-range weapons.

What’s even more striking than most striking is that it was also coincident with human domestication of wolves.  Dogs may have played an integral role in enabling humans to out-compete Neanderthals.   Humans began the process of shrinking wolves between 20,000 and 40,000 years ago.

New data from ancient dogs indicates that dogs became distinct from wolves between 20,000 and 40,000 years ago, researchers report July 18 in Nature Communications.Dogs then formed genetically distinct eastern and western groups 17,000 to 24,000 years ago, the researchers calculate. That timing and other genetic data point to dogs being domesticated just once.

Science News

As early as 10,000 years ago dogs in the Americas were so valued as companions and fellow hunters that at least three of them were given proper burials rather than being eaten:

WASHINGTON — A trio of dogs buried at two ancient human sites in Illinois lived around 10,000 years ago, making them the oldest known domesticated canines in the Americas.

[…]

Ancient dogs at the Midwestern locations also represent the oldest known burials of individual dogs in the world, said Perri, of Durham University in England. A dog buried at Germany’s Bonn-Oberkassel site around 14,000 years ago was included in a two-person grave. Placement of the Americas dogs in their own graves indicates that these animals were held in high regard by ancient people.

An absence of stone tool incisions on the three ancient dogs’ skeletons indicates that they were not killed by people, but died of natural causes before being buried, Perri said.

[…]

Science News

So… Yes, “humans have been driving a global reduction in mammal size for thousands of years” and it had nothing to do with fossil fuels, CO₂ or Gorebal Warming… Nor would renewable energy sources or a Tesla in every garage have prevented it.

Some of the big mammals wound up here:

And some wound up here:

The moral of the story: Big animals that chose to help us hunt fared better than those that didn’t.